The present invention relates generally to electric motors incorporating a centrifugal governor mounted to a rotor shaft of the motor, and a starting switch which is allowed to open or close depending on the rotational speed of the rotor shaft.
Many electric motors include governor assemblies which react to changes in the rotational speed of a rotor shaft to open and close an electric switch. Centrifugal governors generally utilize the centrifugal force generated by rotation of the shaft to engage and disengage an electric starting switch. Illustrated in FIG. 9 is a known starting switch assembly 10 and governor assembly 14. The starting switch assembly 10 includes a bracket 18 mounted to the motor end frame 22 with a mounting screw 26, a switch plate 30 mounted to the bracket 18, and a pair of elongated arms or terminal blades 34 and 38 which are supported by switch plate 30 and each of which includes an associated electric contact 42 and 46. An actuator button 50 is attached to arm 34. The governor assembly 14 includes a spring 52 and a pivot plate 54, and the governor assembly 14 is mounted to the rotor shaft 58. During the power off mode, and during start-up and until the shaft 58 reaches a predetermined rotational speed, the spring 52 causes the pivot plate 54 to contact the actuator button 50 to force the arms 34 and 38 and, therefore, the contacts 42 and 46, together to close the electric switch. After the shaft 58 reaches the predetermined rotational speed, centrifugal forces acting on the pivot plate 54 overcome the force of the spring 52 and cause the pivot plate 54 to pivot in a direction (i.e., a counterclockwise direction as shown in FIG. 9) which allows the actuator button 50 to move in a direction that enables the arms 34 and 38 and, therefore, the contacts 42 and 46, to separate to open the electric switch.
With continued reference to FIG. 9, during start-up and until the shaft 58 reaches the predetermined rotational speed, the spring 52 causes the pivot plate 54 to push the actuator button 50 away from the end 62 of the shaft 58, such that the bottom surface 66 of the actuator button 50 is not perpendicular to the centerline 70 of the shaft 58. As can be appreciated, during start-up and operation of the motor, the governor assembly 14 spins with the rotor shaft 58. The spinning action of the governor assembly 14 and the nonperpendicular engagement between the pivot plate 54 and the bottom surface 66 of the actuator button 50 can cause the actuator button 50 to wobble. Wobbling of the actuator button 50 can result in intermittent arcing between the electric contacts 42 and 46. In order to offset the wobbling action of the actuator button 50, it is known to utilize a significant portion of the available stroke or pivotal movement of the pivot plate 54 of the governor assembly 14. In this way, the engagement between the pivot plate 54 and the actuator button 50 prevents the electric contacts 42 and 46 from separating as the actuator button 50 wobbles. It has been observed that a problem with using a significant portion of the available pivotal movement of the pivot plate 54 to offset the wobbling action of the actuator button 50 is that there may not be a sufficient amount of pivotal movement left for the pivot plate 54 to absorb the stack-up tolerances associated with assembly of the electric motor. In other words, during operation of the electric motor, after the shaft 58 has reached a predetermined rotational speed, there may not be a sufficient amount of stroke left in the pivot plate 54 so as to allow the electric starting switch to open, thereby resulting in possible damage to the starting switch or even the electric motor itself. For example, if a significant portion of the available stroke of the pivot plate 54 is reserved to offset the wobbling action of the actuator button 50, and the length of the shaft 58 is near the high end of the allowed tolerance range, after the shaft 58 reaches the predetermined rotational speed, the pivot plate 54 may not move a sufficient amount to allow the electric contacts 42 and 46 to separate. Consequently, careful adjustment is required to obtain the proper relationship between the switch assembly 10 and the governor assembly 14.
To address the foregoing problems and other problems, the present invention provides an electric motor which includes a xe2x80x9cno-adjustxe2x80x9d starting switch. The starting switch according to the present invention does not require any reservation of the available stroke of a pivot plate of a governor assembly to offset wobbling of an actuator button because, for one reason, the actuator button is prevented from wobbling. One advantage of such an arrangement is that the entire available stroke of the pivot plate of the governor assembly can be used to offset the stack-up tolerances of the motor assembly.
According to one aspect of the present invention, a switch actuator button remains substantially parallel to the centerline of a rotor shaft at all times. As a result, there is minimal wobbling of the actuator button during motor start-up and motor operation. In one embodiment of the present invention, the actuator button is not secured to either terminal blade of the starting switch, so that the actuator button is better able to stay substantially parallel to the centerline of the rotor shaft. In another embodiment of the present invention, the actuator button is constrained for movement parallel to or along the centerline of the rotor shaft to further ensure that the actuator button will not wobble during motor start-up and motor operation. Preferably, a switch housing is provided to simplify the placement of the starting switch into the electric motor, and the switch housing includes an aperture through which the actuator button is slidably positioned, so that during rotation of the rotor shaft, the actuator button is constrained for movement along the centerline of the shaft.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features.